The proposed research is directed toward gaining a better understanding of the organization of DNA sequences within chromosomes of eukaryotes and of how this organization may change during the course of evolution. The virilis group of Drosophila will be the set of organisms studied. Members of this group comprise an exceptionally favorable system since the evolutionary relationships of these species are well defined. Highly repetitive satellite DNA sequences, moderately repetitive DNA sequences, and unique DNA sequences will be isolated, and comparisons will be made among these types of sequences from different species. The techniques involved with this work will include hydroxyapatite chromatography, in situ hybridization, filter hybridization, DNA-excess hybridization and cloning of DNA. Some of the experiments planned will attempt to answer such questions as (1) whether satellite DNA sequences homologous to those in one species are dispersed among the DNA of another species; (2) whether there are homologies among satellite DNAs from the same or different species; and (3) what the relative rates of divergence are among highly repetitive, moderately repetitive, and unique DNA sequences.